(1) Field of the Invention
This invention relates to photopolymerizable compositions, and to methods of producing positive and negative polymeric images from these compositions.
(2) Description of the Prior Art
Conventional polymeric imaging systems are of the negative polymeric image type; that is, the photopolymerizable coating is exposed to radiation through a process transparency and a negative polymeric image is produced. By "negative polymeric image" is meant a polymeric image which corresponds to the transparent portions of the process transparency.
Recently systems which provide a positive polymeric image have been developed. One such system is described by Nebe in U.S. Pat. No. 4,029,505. In this system
(a) a substrate is coated with a photopolymerizable composition containing
(1) a nongaseous, ethylenically unsaturated, polymerizable compound, PA1 (2) 0.001 to 1.0 part by weight per part of polymerizable compound of an organic, radiation-sensitive, free-radical generating system, and PA1 (3) 0.1 to 10% by weight, based on the photopolymerizable composition, of a nitroso dimer PA1 (1) an ethylenically unsaturated monomer, PA1 (2) a photopolymerization initiator system comprising a photo-oxidant dye and a reducing agent for the dye, and PA1 (3) an ionizable desensitizing agent having a nitrobenzyl moiety in combination with a carboxylate group, PA1 N-phenyl-N-methylacrylamide, PA1 N-vinylphthalimide, PA1 diacetone acrylamide, PA1 N-vinylsuccinimide, PA1 p-xylylene diacrylate, PA1 1,4-bis(2-acryloxyethyl)benzene, PA1 pentaerythritol triacrylate, PA1 4-acryloxybenzophenone, PA1 4-methacryloxybenzophenone, PA1 N-(2-acryloxyethyl)succinimide, PA1 1,1,1-trimethylolpropane triacrylate, PA1 pentaerythritol tetraacrylate, PA1 triethylene glycol diacrylate, PA1 triethylene glycol dimethacrylate, PA1 1,1,1-trimethylolpropane trimethacrylate, PA1 4-acryloxydiphenylmethane, PA1 N-(2-acryloxypropyl)succinimide, PA1 2,4-diacryloxybenzophenone, PA1 4-(.alpha.,.alpha.-dimethylbenzyl)phenyl acrylate, PA1 3-acryloxybenzophenone, PA1 2-acryloxybenzophenone, PA1 2-acryloxy-4-octyloxybenzophenone,
(b) the photopolymerizable coating is exposed through a process transparency to radiation at least some of which has a wavelength less than 3400 A whereby the nitroso dimer is dissociated to a polymerization-inhibiting nitroso monomer in the radiation-struck areas, and
(c) a greater portion of the photopolymerizable coating is exposed to radiation substantially limited to wavelengths greater than 3400 A whereby a positive polymeric image is formed in the areas struck by the second radiation but not struck by the first imagewise radiation.
Although this system can produce excellent positive polymeric images, a limited time factor is involved. The second exposure must be carried out soon after the first exposure since the nitroso monomer formed during the first exposure can recombine to form inactive nitroso dimer. It would also be desirable to have a system which is not dependent on exposure to such short wavelength radiation.
Another system for producing positive polymeric images is described by Margerum in U.S. Pat. No. 3,556,794. In this system, a photopolymerizable material comprising
is imagewise exposed to radiation in the wavelength range of 2000-4000 A, thereby inhibiting polymerization by the desensitization of the initiator system, and exposing the photopolymerizable material to radiation in the wavelength range of 3800 to 7200 A, thereby producing a polymeric image in the area not exposed to the imagewise radiation. This system suffers from the drawback that the desensitizing effect of the desensitizing agent is only temporary. Margerum states that the desensitization lasts "for up to about fifteen or more minutes". In Example 1 the second exposure follows the imagewise exposure by 30 seconds.